High-voltage digital power amplifiers or class D amplifiers like AMS AS3911 are known and used in radio frequency identification (RFID) devices like RFID readers to communicate with active or passive transponders. In a typical application a passive transponder or tag stores product identification of a product to which it is attached and the reader is used to obtain this product information. The reader is powered and generates a magnetic field emitted by its RFID antenna. When the reader and the tag are within close proximity of each other, the reader generated magnetic field is induced into the RFID antenna of the tag and used to power the passive tag. The tag also has a transceiver to receive the signal from the reader and to transmit a response back to the reader.
There are standards like ISO/IEC18000-3 or ISO/IEC 14.443 Type A and B or ISO15.693 or ECMA-340 13.56 MHz Near Field Communication (NFC) or company standards like Felica from company Sony that define protocols and types of modulation used to transmit information between the tag and the reader. Some or all of these standards define that the reader transmits digital data to the tags as an analogue signal by changing the magnitude of its transmitted power. The high-voltage power amplifier is used to process the digital data in a low voltage (typ. 3.6V or 5V) domain into an amplified analogue signal with the transmission resonance frequency to drive the RFID antenna in resonance to output maximal power of the substantial sinusoidal output current. The substantial sinusoidal output current of the output signal is needed to satisfy the spurious emission levels required by regulations to avoid noise in other frequency ranges.
FIG. 1 shows the transmission part of a state of the art high-voltage digital power amplifier 1 within a reader that comprises an integrated circuit IC1 that processes digital data to be transmitted to a tag. An adaption circuit 2 of discrete components connected to a first transmission output pin 3 and a second transmission output pin 4 to adapt the output signal of the integrated circuit IC1 and to feed a substantial sinusoidal output current I with a transmission resonance frequency to RFID antenna 10. The digital data as output signal of the integrated circuit IC1 are provided as 0 Volt for bit “0” or “low” and 3.6 Volt to 5 Volt for bit “1” or “high” between the first transmission output pin 3 and the second transmission output pin 4 of the integrated circuit IC1. Filter means 5 are connected to the transmission output pins 3 and 4 to filter the rectangular digital output signal into a substantial sinusoidal signal provided at output connections 6 and 7 of the filter means 5. Resonance means 8 are connected to the output connections 6 and 7 of the filter means 5 and an Ohmic resistance 9 is arranged parallel to RFID antenna 10 and connected to output connections 11 and 12 of resonance means 8.
Resonance means 8 transform the signal into the substantial sinusoidal output current I with the transmission resonance frequency of RFID antenna 10 to output maximal power into the magnetic field HF to be received by the tag. The Ohmic resistance 9 forms the load resistor and generates a substantial sinusoidal output voltage U for RFID antenna 10.
Drawback of this known high-voltage digital power amplifier is the high number of discrete components of the adaption circuit 2 connected to integrated circuit IC1 what increases the size and manufacturing costs of the digital power amplifier. It is substantially not possible to integrate these components into the integrated circuit IC1 of the digital power amplifier 1 due to the high inductive reactance needed for inductors L0a and L0b in filter means 5 and the high capacity needed for the capacitors C0a and C0b in filter means 5 and capacitors C1a and C1b in resonance means 8.